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Hanging Ball, Electric Fields

  1. Jan 17, 2008 #1
    1. The problem statement, all variables and given/known data
    http://session.masteringphysics.com/problemAsset/1001949/13/knight_Figure_25_69.jpg
    An electric field 1.00×10^5 N/C causes the ball in the figure to hang at an angle. What is theta?



    2. Relevant equations
    E = F/q
    F = mg

    3. The attempt at a solution
    The ball isn't accellerating, so it is in equilibrium. Sum of all forces is 0, so the tension force balences gravity + electric field.

    E = F/q
    F = Eq
    F = 100000*(25*0.000001)
    Fx = 2.5

    F = mg
    F = (0.002)(9.81)
    Fy = 0.1962

    Ft = Sqrt(Fx^2 + Fy^2)
    Ft = 2.5076870697916038024546202698347

    tan(theta) = Opp/Adj
    tan(theta) = Fx/Fy
    theta = arctan(2.5 / 0.1962)
    theta = 85.5126 degrees

    This answer is wrong.
    Where is my mistake?
     
    Last edited: Jan 17, 2008
  2. jcsd
  3. Jan 17, 2008 #2

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    You have not given the unit for the electric field, but I'm assuming it's in SI.

    > F = 100000*(25*0.000001)

    The charge is in nC...
     
  4. Jan 17, 2008 #3
    Ah. Yeah, when I copied it over from the online problem, it didn't go.
    E = 1x10^5 N/C
    So I figured I'd convert q into the unit C, therefore get F in the unit N.
     
  5. Jan 17, 2008 #4

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    Did you get the right answer?
     
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